Flat-screen televisions and monitors having liquid crystal displays require backlighting. This backlighting is being increasingly realized using light emitting diodes (LED) rather than cold cathode tubes since LEDs need less energy and have a longer useful life. In FIG. 1, a television 27 having this kind of backlighting is outlined. The light emitting diodes 12 are disposed here in a plurality of light emitting diode strings 2 (LED string) at the upper and lower edges of the television screen 27. In the example, two light emitting diode strings 2 are disposed at the top and at the bottom of a screen 28. The light of the light emitting diodes 12 is distributed uniformly over the entire screen surface by a diffusion disk.
A LED string 2 may consist of a plurality of light emitting diodes 12 that are connected in series and/or in parallel.
In order to obtain uniform illumination over the surface of the screen in this kind of construction, it is absolutely essential for all the light emitting diode strings to have the same brightness. This is achieved in that, with the aid of a balancing circuit, the currents in the individual light emitting diode strings are balanced.
A control circuit having this kind of balancing circuit is shown by way of example in FIG. 2. The control circuit has a plurality of control channels 1 in each of which an LED string 2 is controlled. A balancing transformer 3 acting as a balancing device is disposed in each control channel 1. The primary windings 4 of all the transformers 3 are connected in series and connected to a bridge circuit 5 acting as an AC voltage source. The bridge circuit 5 is supplied via a power factor correction controller 6 (PFC) with a DC voltage. The PFC 6 and bridge circuit 5 are connected to a control unit 7 that regulates, for example, the output voltage of the PFC 6. The LED strings 2 are each connected via a bridge rectifier 13 to the secondary windings 8 of the balancing transformers 3. The lamp current in all control channels 1 is identical and corresponds to the active current in the primary windings 4.
Dimming the light emitting diode strings is made possible in this circuit by varying, for example, the primary-side current, for instance, through modulation of the primary voltage or current source (burst mode dimming) However, since all the control channels are balanced, they will be dimmed identically.
In some applications, however, it is necessary for the control channels in burst mode dimming to be switched on and then off again with a slight time delay, as is shown in the time diagram in FIG. 3. Dimming the lamps while maintaining current balancing is then only possible, for example, if each control channel has its own regulated power supply. A control circuit of this kind, however, is complex and expensive.
It is thus an object of the invention to create a simple control circuit that allows the control channels to be switched with a time delay while at the same time maintaining balancing of the lamp currents in the individual lamp channels.